Expanded packing products, which are hardened polymer foams, enjoy widespread use as loose-fill packing material. Loose fill packing based on polystyrene currently constitutes the majority of the market for such expanded foam packing products. The use of polystyrene for this purpose has enjoyed several advantages: (1) such foams are relatively inexpensive; (2) very low density foams can be made therefrom; (3) the foams are quite uniform in their physical properties; (4) they have good resiliency; (5) they have a pleasing appearance; and (6) they are easily conveyed in air. Thus, while the advantages of polystyrene packing are many, the disadvantages of the material are significant: (1) the manufacture of polystyrene foams involves volatile hydrocarbon blowing agents, such as pentane, which presents a fire hazard, or halocarbons, which are environmentally unacceptable; and (2) such foams are not biodegradable.
More recently, substantial progress has been made toward the use of starch-based foamed packing, which has the distinct advantage of being biodegradable both in sunlight and in the difficult conditions that exist in compost heaps, landfills and other common disposal means. Such starch-based packing is customarily based on formulations of a water-soluble polymer, such as poly(vinyl alcohol), with various kinds of starch. Though starch-based packing has many physical properties comparable to polystyrene-based packing materials, it has heretofore been quite difficult to obtain comparably low densities at economical material costs. Low densities are, of course, quite important from three different viewpoints: (1) minimization of the weight of packaged goods; and (2) ease of conveying the foamed packing from storage to the packing site, and (3) lower per unit volume cost of packing materials. In particular, the flow characteristics of starch-based packing have been found to be a function of the density of the packing particles. Thus, low density particles are much easier to handle in pneumatic conveying devices than higher density particles.
As a result of these differences in properties, there is considerable incentive to create starch-based packing materials having lower densities than those previously obtained, but which are nevertheless comparable to those obtainable with polystyrene packing.